Allows plotting GridSpaces inside Layouts in bokeh

[philippjfr]

This PR makes the bokeh GridPlot and LayoutPlot fully composable so you can now plot multiple grids together. Here's two grids and a regular element in a layout:

%%opts Image [height=260]
(hv.GridSpace({(i, j): hv.Curve(range(i+j)) for i in range(1, 3) for j in range(2,4)}) +\
 hv.GridSpace({(i, j): hv.Curve(range(i+j)) for i in range(1, 3) for j in range(2,4)}) +
 hv.Image(np.random.rand(10,10)))

The Composing_Data tutorial now also works for both matplotlib and bokeh:

Related: #381

[philippjfr]

Hmm, the drop in coverage is odd. Will have to look into it.

[philippjfr]

Now fixed, ready to review/merge.

[jlstevens]

Unfortunately there now seem to be merge conflicts. Happy to merge once that is resolved and the tests have passed.

[philippjfr]

First time I resolved merge issues directly on GitHub, ready to merge when passing again.

[jlstevens]

Looks good and the tests are passing. Merging.